1. Field of the Invention
The present invention relates to an oxide magnetic material having soft magnetism, particularly to a Mnxe2x80x94Zn ferrite and more particularly to a Mnxe2x80x94Zn ferrite suitable for use as a high permeability material used for various inductance elements, impedance elements for EMI countermeasure or the like, a low loss material used for switching power transformers, an electromagnetic wave absorbing material and the like, and a production process thereof.
2. Description of the Related Art
A Mnxe2x80x94Zn ferrite is counted among the typical oxide magnetic materials having soft magnetism. The Mnxe2x80x94Zn ferrite of the prior art usually has a basic component composition containing more than 50 mol % (52 to 55 mol % on the average) Fe2O3, 10 to 24 mol % ZnO and the remainder consisting of MnO. And the Mnxe2x80x94Zn ferrite is usually produced by mixing the respective material powders of Fe2O3, ZnO and MnO in a prescribed ratio, subjecting to the respective steps of calcination, milling, component adjustment, granulation, pressing and the like to obtain a desired shape, then conducting sintering treatment in which the resulting product is kept at 1200 to 1400xc2x0 C. for 3 to 4 hours in a reducing atmosphere in which a partial pressure of oxygen is limited to a low level by supplying nitrogen. Incidentally, the reason why the Mnxe2x80x94Zn ferrite is sintered in the reducing atmosphere is that when it contains Fe2O3 exceeding 50 mol % and is sintered in the air, densification is not attained sufficiently thereby failing to obtain excellent soft magnetism, and that although Fe2+ formed by the reduction of Fe3+ has positive crystal magnetic anisotropy and cancels negative crystal magnetic anisotropy of Fe3+ thereby enhancing soft magnetism, such a reducing reaction cannot be expected if sintering is conducted in the air.
Incidentally, it has been known that the above-mentioned densification depends on the partial pressure of oxygen in the temperature rise at the time of sintering and the above mentioned formation of Fe2+ depends on the oxygen in the temperature fall after sintering, respectively. Therefore, when the setting of the partial pressure of oxygen at the time of sintering is wrong, it becomes difficult to ensure an excellent soft magnetism. Thus, in the prior art, the following expression (1) was experimentally established and the partial pressure of oxygen at the time of sintering has been conventionally controlled strictly in accordance with this expression (1).
log PO2=xe2x88x9214540/(T+273)+bxe2x80x83xe2x80x83(1)
where T is temperature (xc2x0C.), PO2 is a relative partial pressure of oxygen, wherein PO2=Pxe2x80x2O2/Ptotal, Pxe2x80x2O2 is the absolute partial pressure of oxygen (Pa), and Ptotal is the absolute total pressure (Pa), and b is a constant. The constant b has been set at about 7 to 8. The fact that the constant b is set at 7 to 8 means that the partial pressure of oxygen during sintering must be controlled at a narrow range, which makes the sintering treatment very troublesome thereby increasing the production costs.
On the other hand, when the Mnxe2x80x94Zn ferrite is used as a magnetic core material, eddy current flows at a higher frequency region, resulting in a larger loss. Therefore, to extend an upper limit of the frequency at which the Mnxe2x80x94Zn ferrite can be used as a magnetic core material, its electrical resistance must be set as high as possible. However, the electrical resistance in the above-mentioned usual Mnxe2x80x94Zn ferrite has values smaller than 1 xcexa9m due to the transfer of electrons between the above-mentioned Fe3+ ions and Fe2+ ions and a frequency which is available for application is limited to about several hundred kHz maximum. Thus, in a frequency region exceeding this limit, permeability (initial permeability) is remarkably lowered and the properties of the soft magnetic material are lost.
The present invention has been made in consideration of the above-mentioned conventional problems. The present invention has objects to provide a Mnxe2x80x94Zn ferrite which has a high electrical resistance and can sufficiently satisfy applications in a high frequency region exceeding 1 MHz, and to provide a production process thereof in which such Mnxe2x80x94Zn ferrite can be obtained easily and at low costs.
The present inventors recognized in a series of researches related to the Mnxe2x80x94Zn ferrite that even if Fe2O3 content is limited to 50.0 mol % or less, the Mnxe2x80x94Zn ferrite has a high electrical resistance by allowing suitable amounts of TiO2 and/or SnO2 to be contained and further a suitable amount of CuO to be contained as desired and can sufficiently satisfy applications in a high frequency region exceeding 1 MHz, and have already disclosed the above in Japanese Patent Application No. Hei 11-29993 and Japanese Patent Application No. Hei 11-29994 (both applications are unpublished).
The inventions in the above-mentioned applications filed are made under the conviction that Fe2+ can be formed by allowing the Mnxe2x80x94Zn ferrite to contain Ti and/or Sn even when the Mnxe2x80x94Zn ferrite is sintered in the air or in an atmosphere containing some amount of oxygen, which is derived from the findings that iron components in the Mnxe2x80x94Zn ferrite exist as Fe3+ and Fe2+ and that Ti and Sn receive electrons from this Fe3+ to form Fe2+. Further, in the inventions of the above-mentioned filed applications, the content of TiO2 and/or SnO2 in the basic component composition is limited to 0.1 to 8.0 mol % for controlling the amount of Fe2+ formed so that the coexistence ratio of Fe3+ to Fe2+ is optimized to offset positive and negative crystal magnetic anisotropy, whereby an excellent soft magnetism can be obtained. Further, since a number of Ti4+ and Sn4+ ions which have stable valences exist under the conditions, even if Fe2O3 content is limited to a low level, an exchange of electrons between Fe3+ and Fe2+ is substantially blocked. Thus, an electrical resistance remarkably higher (about 103 times) than conventionally can be obtained.
The present inventors have made the invention by finding out in a series of researches related to the Mnxe2x80x94Zn ferrite that the initial permeability, particularly the initial permeability in a high frequency region is further enhanced by allowing one or more from CoO, NiO and MgO to be contained in a suitable amount as additive to a basic component composition in which Fe2O3 content is limited to 50.0 mol % or less and in which TiO2 and/or SnO2 is contained in a suitable amount as described above.
That is, a Mnxe2x80x94Zn ferrite according to one aspect of the present invention to attain the above-mentioned objects is characterized in that the basic component composition contains 44.0 to 50.0 mol % Fe2O3, 4.0 to 26.5 mol % ZnO, 0.1 to 8.0 mol % one or two from TiO2 and SnO2 and the remainder consisting of MnO, and further contains 0.01 to 2.00 mass % one or more from CuO, NiO and MgO as additive.
Further, a Mnxe2x80x94Zn ferrite according to another aspect of the present invention is characterized in that the basic component composition contains 44.0 to 50.0 mol % Fe2O3, 4.0 to 26.5 mol % ZnO, 0.1 to 8.0 mol % one or two from TiO2 and SnO2, 0.1 to 16.0 mol % CuO and the remainder consisting of MnO, and further contains 0.01 to 2.00 mass % one or more from CoO, NiO and MgO as additive.
The Mnxe2x80x94Zn ferrite according to the present invention is characterized in that Fe2O3 content is limited to 50.0 mol % or less as described above. However, since too little Fe2O3 content leads to reduction in the saturation magnetization or initial permeability, at least 44.0 mol % Fe2O3 is adapted to be contained.
ZnO affects the Curie temperature and saturation magnetization. If ZnO is contained in a significant amount, the Curie temperature is lowered, resulting in practical problems. On the other hand, if ZnO is contained in too small an amount, the saturation magnetization is reduced. Thus, ZnO content is preferably controlled to the above-mentioned range of 4.0 to 26.5 mol %.
CuO has an effect to enable the Mnxe2x80x94Zn ferrite to be sintered at a low temperature. However, if the CuO content is too small, the effect is small. On the other hand, if the CuO content is too large, the initial permeability is reduced. Thus, CuO content is preferably controlled to the above-mentioned range of 0.1 to 16.0 mol %.
Since all of CoO, NiO and MgO are metal oxides having magnetism, they are solid-dissolved in the spinel lattices of the Mnxe2x80x94Zn ferrite and impart good influences to the magneto-striction, crystal magnetic anisotropy, and induced magnetic anisotropy, respectively. However, if their content is small, the effect is small. On the other hand, if their content is too large, the reduction of the initial permeability occurs. Thus, CoO, NiO or MgO content is preferably controlled to the above-mentioned range of 0.01-2.00 mass %.
Since in the Mnxe2x80x94Zn ferrite according to the present invention, Fe2O3 content is limited to 50 mol % or less as described above, even if the Mnxe2x80x94Zn ferrite is sintered in the air or in an atmosphere containing some amount of oxygen, densification is sufficiently attained and desired soft magnetism can be obtained.
That is, a production process according to one aspect of the present invention to attain the above-mentioned objects is characterized in that mixed powder whose components are adjusted so as to compose the above-mentioned Mnxe2x80x94Zn ferrite is pressed, then sintered and cooled in the air.
Further, a production process according to another aspect of the present invention is characterized in that mixed powder whose components are adjusted so as to compose the above-mentioned Mnxe2x80x94Zn ferrite is pressed, then sintered and cooled in an atmosphere with the partial pressure of oxygen obtained by using an optional value in a range of 6 to 21 as the constant b in the aforementioned expression (1).
In this case, when a value larger than 21 is selected as the constant b in the expression (1), the resulting atmosphere becomes substantially the same as the air. Thus, it makes no sense to define the partial pressure of oxygen. Further, if this constant b is smaller than 6, electrical resistance becomes too low, whereby initial permeability at a high frequency is deteriorated.
In the production of a Mnxe2x80x94Zn ferrite, raw material powders of Fe2O3, ZnO, TiO2 and/or SnO2, CuO, MnO and the like used as main components are previously weighed to match the above defined basic component composition, and are mixed. Then, the mixed powder is calcined and finely milled as required. The calcining temperature is slightly different depending upon the target composition, and an appropriate temperature can be selected within a range of 800 to 1000xc2x0 C. A general-purpose ball mill can be used for fine milling of the mixed powder. Further, powder of CoO, NiO or MgO is added to the fine mixed powder in a prescribed amount (0.01 to 2.00 mass %) as the additive and mixed to obtain mixed powder having the target composition. Then, the mixed powder is granulated and pressed in accordance with a usual ferrite production process, and sintered at 900 to 1400xc2x0 C. Incidentally, a process of adding a binder such as polyvinyl alcohol, polyacrylamide, methyl cellulose, polyethylene oxide, glycerin, or the like can be used for the granulation, and a process of applying pressure of, for example, 80 MPa or more can be used for the pressing.
The above-mentioned sintering and cooling after the sintering can be conducted in the air or in an atmosphere with the partial pressure of oxygen defined based on the aforementioned expression (1) where the constant b is within a range of 6 to 21. However, when they are conducted in an atmosphere containing oxygen, it is desirable to control the oxygen by allowing an inert gas such as nitrogen or the like to flow into a sintering furnace. In this case, an optional value in a wide range of 6 to 21 can be selected as the constant b given in the expression (1). Therefore, the control of the partial pressure of oxygen can be made easily.
Since the Mnxe2x80x94Zn ferrite thus obtained contains TiO2 and/or SnO2 as main components while containing 50 mol % or less Fe2O3, electrical resistance is remarkably increased (about 103 times) as compared to conventional Mnxe2x80x94Zn ferrite.
Further, generally, the limit of the initial permeability xcexc in soft magnetic ferrite is inversely proportional to a frequency f (MHz) at which the ferrite is used, and is estimated with a value obtained by the expression (2) given below, but since the Mnxe2x80x94Zn ferrite according to the present invention contains CoO, NiO or MgO as additive in a prescribed amount, an initial permeability xcexc of about 2000 at 1 MHz or about 200 at 10 MHz can be obtained as estimated. Thus, the present Mnxe2x80x94Zn ferrite can be ideal for use in a magnetic core material and an electromagnetic wave absorbing material for a high frequency exceeding 1 MHz.
xcexc=K/f(K=1500 to 2000)xe2x80x83xe2x80x83(2)